North Korea: What would happen if Kim Jong-un ordered nuclear explosion in Pacific

North Korea is believed to be planning to test a powerful nuclear missile: Korean Central News Agency via AP

North Korea may be planning one of the most powerful nuclear explosions in history, if the nation's foreign minister is to be believed.

Ri Yong-ho, the foreign minister of the isolated nation, reportedly told journalists that the nation's leader Kim Jong-un is considering such a test blast.

“It could be the most powerful detonation of an H-bomb in the Pacific,” Mr Ri told reporters at the United Nations in New York on Thursday, according to a story by South Korea's Yonhap News Agency.

“We have no idea about what actions could be taken as it will be ordered by leader Kim Jong Un.”

The suggestion came in response to bellicose rhetoric exchanged between US President Donald Trump and Mr Kim.

In a speech before the UN General Assembly on Tuesday, Mr Trump called Mr Kim a suicidal “rocket man” and threatened to “totally destroy” North Korea if the US is “forced to defend itself or its allies.”

Mr Kim allegedly responded with a written statement, in which he called Mr Trump a “mentally deranged US dotard” and said that “a frightened dog barks louder.”

Many experts have denounced Mr Trump's speech, suggesting his words could provoke Mr Kim to take dramatic action.

“Trump is basically creating audience costs for Kim to back down,” Jeffrey Lewis, director of the East Asia Nonproliferation Program at the Middlebury Institute of International Studies at Monterey, told Vox. “If you dare Kim, it creates pressure for him to respond with his own provocation.”

North Korea has set off several powerful nuclear test blasts in recent years, but they all occurred deep inside a mountain. A nuclear explosion in the air, on the ground, underwater, or in space has not happened in decades.

If the nation sets off an above-ground nuclear explosion — and the most powerful ever detonated in the Pacific — the Cold War's rich history of test blasts suggests what might happen.

The US, Russia, China, and other countries have carried out more than 2,000 nuclear test blasts since 1945.

More than 500 of these explosions occurred on soil, in space, on barges, or underwater. But most of these happened early in the Cold War — before the risks to innocent people and the environment were well-understood. Nearly all countries now ban nuclear testing.

The problem with nuclear test explosions is that they create radioactive fallout. Space detonations come with their own risks, including a more widespread electromagnetic pulse.

Only a fraction of a nuclear weapon's core is turned into energy during an explosion; the rest is irradiated, melted, and turned into fine particles. This creates a small amount of fallout that can be lofted into the atmosphere and spread around.

But the risk of fallout vastly increases when a blast occurs close to the ground or water. There, a nuclear explosion can suck up dirt, debris, water, and other materials, creating many tons of radioactive fallout — and this material rises high into the atmosphere, where it drifts for hundreds of miles.

This kind of Cold War-era fallout killed scores of innocent people in the Pacific, including Japanese fishermen, and is still causing cancer and health problems around the world today.

Mr Ri did not specify where or how high North Korea's hypothetical Pacific “H-bomb” test might occur. However, the foreign minister did reportedly suggest it could be the most powerful ever detonated in the Pacific.

If this is not a matter of imprecise wording, such a blast would exceed the US' strongest-ever nuclear test explosion.

On March 1, 1954, the US military set off the “Shrimp” thermonuclear device on a platform in the Bikini Atoll in the Marshall Islands (about 2,300 miles southeast of Japan and 2,700 miles southwest of Hawaii).

This was part of the US military's Castle Bravo test series, and the blast was equivalent to exploding 15 million tonnes of TNT, or roughly 1,000 times as powerful the US attack on Hiroshima that inflicted some 150,000 casualties.

While the military considered Shrimp and Bravo a success, its repercussions were disastrous. Researchers underestimated the device's explosive power by nearly three-fold — and many were nearly killed when an artificial earthquake shook their concrete observation bunker 20 miles away.

Author and film producer Eric Schlosser, writing in his book “Command and Control: Nuclear Weapons, the Damascus Accident, and the Illusion of Safety”, captures the raw power of the blast through the perspective of scientist Bernard O'Keefe:

“About ten seconds after Shrimp exploded, the underground bunker seemed to be moving. But that didn't make any sense. The concrete bunker was anchored to the island, and the walls were three feet thick.

”'Is this building moving or am I getting dizzy?' another scientist asked. 'My God, it is,' O'Keefe said. 'It's moving!'

“O'Keefe began to feel nauseated, as though he were seasick, and held on to a workbench as objects slid around the room. The bunker was rolling and shaking, he later recalled, 'like it was resting on a bowl of jelly.' The shock wave from the explosion, traveling through the ground, had reached them faster than the blast wave passing through the air.”

The scientists ultimately escaped alive, but Marshall Islanders located 100 miles from the blast were not so lucky.

Shrimp's four-mile-wide fireball destroyed about 200 billion tons of Bikini Atoll coral reef, turning much of it into radioactive fallout that spread all over the world. The worst of it sprinkled over atolls to the east, killing many people by causing radiation sickness.

Today, the 250-foot-deep, 1-mile-wide crater left by the blast is visible from space.

If North Korea decides to blow up a hydrogen or thermonuclear device — and the most powerful in the Pacific — we could only hope it is not close to the ground.

All of these scenarios assume North Korea sets off a thermonuclear device in a controlled way — via airplane, barge, balloon, or some kind of stationary platform.

But the risk to people also largely depends on whether or not North Korea launches a nuclear warhead on an intercontinental ballistic missile or a shorter-range rocket, such as one launched from a submarine.

If successful, such a missile test would show North Korea has miniaturised its weapons. And if the blast appears to be caused by a hydrogen bomb, it would show North Korea could pull off a devastating thermonuclear strike on US soil.

But missiles are prone to failure in multiple ways, especially those in early development. A North Korean ICBM tipped with a nuclear warhead might miss its target by a significant distance, or explode en route. This could lead to detonation in an unintended place and altitude.

This is especially true if the missile has no self-destruct capability — ICBMs maintained by the US don't. In that case, only hacking the missile's software in mid-air, or destroying it with another weapon, could stop the launch.

“The stakes and heat in this conflict have not been this high since the Korean War,” Tristan Webb, a senior analyst for NK News, said in a story published by the outlet on Friday. “Kim Jong Un said in July that the ... showdown was entering its final phase. He appears psychologically prepared for conflict.”